**4. Alloying of AlCrN**

The alloying (doping) of (AlCr)N is amongst the most important approaches used to develop and adapt these coatings. The addition of at least one alloying element (such as Si, B, C, O, W, Y and others) into (AlCr)N coatings is used to further optimise properties and performance. Through alloying, the base coating may be improved in terms of increased hardness, higher oxidation resistance, extended thermal stability, higher wear resistance and lower friction values. The main directions in the development of alloyed (AlCr)N coatings will be reviewed in this section. Figure 19 shows an overview of alloying concepts. Metals, metalloids, and oxygen are added as single elements or in combination with others.

Aspects of environmental sustainability have not yet been discussed in the scientific and engineering community until now. These important ecological aspects, such as selecting specific alloying elements to minimise environmental impacts in the production of the coating, the use of the coating and the recycling, were overlooked. In general, the addition of O, C and Si seems to offer the most sustainable solutions. An overview of different alloying directions that have been reported is given in Table 2.

**Figure 19.** Directions of alloying for (CrAl)N and (AlCr)N designed to tune selected properties.


metalloid component.

#### *Coatings* **2021**, *11*, 344


**Table 2.** *Cont.*

*Coatings* **2021** , *11*, 344


**Table 2.** *Cont.*
